Simon Hughes is party president and also the Lib Dem spokesman on Ministry of Justice issues.
The North Southwark and Bermondsey MP stood unsuccessfully against Ken Livingstone for London Mayor in May 2004, coming third.
He was also a rival to Charles Kennedy for the party leadership in 1999.
Mr Hughes came third, to Sir Menzies Campbell and Chris Huhne, when he ran again for the leadership in 2006.
He is an environmentalist and anti-nuclear campaigner, as well as being an opponent of both abortion and euthanasia.
Mr Hughes is well known around Parliament for the bright yellow London taxi he drives and for his frequent speeches.